Electrical alternans is a phenomenon that can be observed on an electrocardiogram (ECG) or electrogram recording as beat to beat alternation in the amplitude or morphology of electrocardiogram or electrogram waveforms. Electrical alternans was first recognized by H. Hearing in 1909 and was further characterized by Sir Thomas Lewis in 1910 as occurring in two instances: where the heart rate is fast, but the heart muscle is normal; or where the heart rate is normal, but the patient has a serious heart disease. Electrical alternans is a repolarization or conduction abnormality of the Purkinje fibers or myocardium. Typically alternans occur at elevated heart rates but the threshold heart rate can be lowered by various conditions like ionic disturbances or ischemia.
Repolarization alternans, also known as T-wave alternans (TWA), is defined as beat-to-beat fluctuations in amplitude, polarity or shape of a T wave of an electrocardiogram or electrogram signal. TWA has been associated with propensity to ventricular arrhythmias that result in sudden cardiac death. While seemingly unpredictable, ventricular arrhythmia is now recognized to exhibit features of a chaotic system en route from underlying instability to a disorganized pattern. In this progression, TWA that measure dispersion of recovery serve as a marker of cardiac instability.
Visible macroscopic TWA present in an ECG measured with surface electrodes have been associated with imminent onset of dangerous ventricular arrhythmias, and in some cases may leave limited time to intervene. Microvolt level TWA (mTWA) can be used to assess subtle changes in repolarization that occur far in advance of an arrhythmia. Microvolt level fluctuations in the T wave are not detectable by the unaided human eye, but can be revealed with computer analysis.
Mechanical alternans is a phenomenon of alternating strong and weak beats despite a constant heart rate. This phenomenon has been observed in patients with left ventricular dysfunction and seems to be related to risk for progressive heart failure. Mechanical alternans are often accompanied by electrical alternans.